Aspirin has been used as anti-inflammatoric antipyretic analgesics from of old. It is in general orally administered in form of tablets, granules and so on.
However, due to the intestinal injury by Aspirin, its external application has been recently studied and it has been done to try to make it percutaneously absorb. The results are reported in Japanese Patent Pub. No. 3-72426 in form of ointments for treatment of neuralgia, in Japanese Patent Pub. No. 6-72879 in form of patches containing Aspirin, in Japanese Patent Pub. No. 6-183980 as a stabilizing method of Aspirin in patches containing it. Further, patches containing Aspirin are disclosed in Japanese Patent Pub. No. 8-113531. Techniques such as improvement of transdermal absorption of Aspirin and stabilization of it in plasters are mainly disclosed in these publications. However, any technique to maintain Aspirin stably in ointments for a long term is not disclosed.
The literatures which describe methods for stabilization of Aspirin in preparations except for external preparations, are Japanese Patent Pub. No. 56-32425, Japanese Patent Pub. No. 62-89619, Japanese Patent Pub. No. 4-346930 and so on.
Because Aspirin is readily hydrolyzed even in the presence of small amount of water and furthermore, by depending on a kind of additives the hydrolysis is accelerated, in these literatures in order to avoid to contact with the additive, it is disclosed to use the protective layer consisting of sucrose, or to use binders in which water was excluded as much as possible degree and to add a hydrogenated oil as a lubricant. However, it is hardly possible to apply such techniques to ointments.
As such, in external preparations of Aspirin, the technique to secure the stability of Aspirin in preparing ointments has not been shown.